Man, My Neighbor Totoro is the best Studio Ghibli movie ever produced. It's a shame the studio itself is currently on a kind of hiatus for something.
I'm partial to Spirited Away, personally. I think it's the best animated film ever. I also recently re-watched Princess Mononoke, and I think it's underrated. It's got to be the second best Ghibli film. It's really very powerful.